1. Field of the Invention
The present invention relates to an acoustic monolithic power IC (or semiconductor integrated circuit) and an acoustic system using the same.
2. Description of the Prior Art
An acoustic system to be carried on a vehicle such as an automobile may be composed of front speakers and rear speakers. The sound volumes of the front and rear speakers can be changed as desired so that the acoustic reproduction can be accomplished under any desired conditions, e.g., mainly by the front speakers, mainly by the rear speakers, or in the same volume by the front and rear speakers. According to one prior art method, the front and rear speakers are driven by output amplifiers which have their levels adjusted at their input sides and which are made independent of one another. According to another prior art system, on the other hand, the front and rear speakers are connected with the output of one output amplifier through a plurlity of level adjusting variable attenuators so that they may be driven. The sound volumes of the front and rear speakers can be varied by adjusting the variable attenuators. The latter system has an advantage that the number of the output amplifiers can be minimized.
The aforementioned variable attenuators are, however, relatively expensive because they are required to admit relatively high currents. In case, moreover, a BTL (Balanced Transformer Less) amplifier is used as the output amplifier to increase the output power for an identical supply voltage, the variable attenuators, which are relatively expensive, as noted above, have to be connected with one pair of output terminals, respectively, with a view of adjusting the sound volumes of the front and rear speakers. For the stereophonic acoustic reproduction, it is necessary that two sets of the aforementioned output amplifiers and volumes be used.